Add dot-syntax support.

[inamiy]

It'll be lovely if this library can support dot-syntax :)

[bklimt]

This is a very neat trick. I like the way it lets you write a sequence of tasks without having a jumble of "[[[[" at the beginning. Unfortunately, I don't think we'll be able to merge this pull request. Our primary concerns are:

1. The Objective-C community will likely object to this non-standard syntax that feels a bit like trying to imitate the Java style.
2. Having continueWithBlock: mutate the BFTask that it is called on in an observable way is unintuitive, and does not feel like it's in the spirit of what a Task represents.
3. There is a known issue with including categories in libraries that causes linker failures when not using the -ObjC flag. However, using -ObjC has its own set of problems. So we are going to avoid using categories in Bolts for the time being. For more information, see https://developer.apple.com/library/mac/qa/qa1490/_index.html

Thanks again for taking the time to put this together.

[inamiy]

All right. Thanks for review.